1. Field of the Invention
The present invention relates to a honeycomb filter, and a manufacturing method of the honeycomb filter. More particularly, it relates to a honeycomb filter which can suppress the increase of an initial pressure loss and also suppress the rise of a pressure loss when a particulate matter is deposited, and also relates to a manufacturing method of a honeycomb filter which can manufacture such a honeycomb filter.
2. Description of Related Art
In gases discharged from internal combustion engines such as diesel engines, various types of combustion apparatuses and the like, a large amount of particulate matter (PM) mainly composed of soot is included. When this PM is discharged as it is to the atmosphere, environmental pollutions are caused. Therefore, a diesel particulate filter (DPF) for collecting the PM is mounted on an exhaust system of an exhaust gas.
As such a DPF, there has been used, for example, a honeycomb structure “including porous partition walls arranged to partition a plurality of cells which become through channels of a fluid (an exhaust gas and a purified gas) and an outer peripheral wall positioned in an outermost periphery, and further including plugged portions in open frontal areas of predetermined cells in an end surface on an inflow side of the fluid (the exhaust gas) and open frontal areas of the remaining cells in an end surface on an outflow side of the fluid (the purified gas)”.
There has been a problem that when the PM in the exhaust gas is collected by using such a honeycomb structure, the PM penetrates into the porous partition walls to clog pores of the partition walls, and a pressure loss rapidly increases sometimes.
A filter has been suggested in which to suppress the increase of a pressure loss, a collecting layer for collecting a PM is disposed on the surfaces of partition walls, and the collecting layer prevents the penetration of the PM into the partition walls, to suppress the rise of the pressure loss (e.g., see Non-Patent Document 1).
Heretofore, as a regulation on an automobile exhaust gas, the regulation by means of a mass of the PM has been performed. In recent years, however, the introduction of the regulation on the number of the PMs has been investigated. In this case, it is necessary to securely collect the PM having small particle diameters. It is known that the PM having the small particle diameters is collected by the surfaces of pores which are present in a filter, mainly by diffusion (e.g., see Non-Patent Document 2).
Heretofore, when the honeycomb structure including the collecting layer is prepared, a wet type coating has been performed by immersing the honeycomb structure into a collecting layer forming slurry (a collecting layer forming raw material) or pouring the collecting layer slurry into cells of the honeycomb structure, to coat porous partition walls (a partition wall parent material) with the collecting layer forming slurry, or a dry type coating has been performed by sucking raw material powder together with air to coat the porous partition walls (the partition wall parent material) with particles, followed by firing, to form the collecting layer. Moreover, when a porous film having smaller pore diameters and smaller thickness than the porous partition walls is formed on the surfaces of the partition walls, it has been necessary to set the particle diameters of ceramic particles constituting the porous film to be smaller than the pore diameters of the partition walls. However, in this method, there has been a problem that the collecting layer forming slurry or the raw material powder penetrates into the pores of the partition walls (the partition wall parent material) of the honeycomb structure, and an initial pressure loss in the case of the circulation of the exhaust gas through the obtained honeycomb structure becomes high.
For example, as a manufacturing method of such a honeycomb structure, there has been suggested a method in which a collecting layer forming slurry prepared by further adding a pore former and water to the same material as that of a honeycomb formed body is sprayed on the honeycomb formed body, to deposit the collecting layer forming slurry on partition walls of the honeycomb formed body, followed by drying and firing, thereby disposing the collecting layer on the honeycomb formed body (e.g., see Patent Document 1).
Furthermore, there has been suggested a method in which a slurry made of a bonding material mainly composed of an inorganic fibrous material longer than pore diameters of porous partition walls and silica or alumina is deposited on the surfaces of the partition walls, followed by drying and firing, thereby forming a porous film (a collecting layer) on surface layers of the partition walls (e.g., see Patent Document 1).
In addition, there has been suggested a method in which particles having an average particle diameter smaller than that of particles constituting partition walls are supplied through one opening end of a honeycomb fired body by a solid-gas two-phase flow, and the particles are deposited in open pores formed by the particles constituting the partition walls and/or spaces among the particles, in surface layer portions of the partition walls, to form a composite region (a collecting layer) (e.g., see Patent Document 2).
[Patent Document 1] WO2008/136232A1
[Patent Document 2] WO2010/110010A1
[Non-Patent Document 1] SAE Technical Paper 2008-01-0618, Society of Automotive Engineers (2008)
[Non-Patent Document 2] SAE Technical Paper 2007-01-0921, Society of Automotive Engineers (2007)
However, in the manufacturing method disclosed in Patent Document 1, a collecting layer is formed by a wet type process such as spray coating or dip coating by use of a collecting layer forming slurry including an inorganic fibrous material which is longer than pore diameters of partition walls. Therefore, the inorganic fibrous materials constituting the collecting layer are aligned with each other, or the inorganic fibrous material and the partition walls (a partition wall parent material) are aligned, thereby decreasing through channels through which a gas and the like penetrate into the collecting layer. This has caused a problem that a pressure loss in an initial state of an obtained honeycomb structure increases. Moreover, such a honeycomb structure also has a problem that the rise of a pressure loss when a PM is deposited (especially, in a deposition initial stage where the PM starts to be deposited) becomes large.
Moreover, in the manufacturing method disclosed in Patent Document 2, particles are deposited in pores of partition walls, and hence through channels through which a gas penetrates into a composite region (a collecting layer) decrease. In consequence, there has been a problem that a pressure loss in an initial state of an obtained honeycomb structure increases. Moreover, such a honeycomb structure also has a problem that the rise of a pressure loss when a PM is deposited (especially, in a deposition initial stage where the PM starts to be deposited) becomes large.